1. Field of the Invention
The present invention relates to linearization of a power amplifier used in a cellular phone and base station, for example, as well as to a low-distortion power amplifier and the like.
2. Description of the Related Art
FIG. 17 shows a conventional predistortion circuit. As shown in FIG. 17, a signal that is input to an input terminal 1701 is divided to two routes by a divider circuit 1703. In the first route, the signal is input to a combiner 1707 via a delay circuit 1704. On the other hand, in the second route, a distortion is generated by a distortion generation circuit 1705 and the signal is input to the combiner 1707 via a vector adjustment circuit 1706. The two signals are combined with each other by the combiner 1707 and a resulting signal is output from an output terminal 1702 and supplied to a power amplifier (not shown). The predistortion circuit generates a signal for suppressing a distortion of the downstream power amplifier by varying the amplitude and phase of a distortion with the vector adjustment circuit 1706, whereby a distortion at the output of the power amplifier is suppressed. The delay time of the delay circuit 1704 is so set as to equalize the delay times of the first and second routes.
However, in particular, where a transmission signal has a wide bandwidth and the power amplifier performs a class-AB operation, the amplitude and phase characteristics of a intermodulation distortion generated by the amplifier lose balance and hence the effect of linearization (for example, suppressing distortion) degrades.
A more detailed description will be made below. It is assumed that the power amplifier is matched in a wide band and that the gain and the pass phase have no deviations in a transmission band. When two signals having different frequencies and the same amplitude are input, the output voltage VO is given by
VO=AO(cos xcfx891t+cos xcfx892t)+BOL cos [(2xcfx891xe2x88x92xcfx892)t+xcfx863L]+BOU cos[(2xcfx892xe2x88x92xcfx891)t+xcfx863U]xe2x80x83xe2x80x83[Equation 1]
where xcfx891 and xcfx892 are the angular frequencies of the input signals, AO is the amplitude component of voltages having angular frequencies xcfx891 and xcfx892 (among the output voltages), BOL and BOU are the amplitude components of third-order intermodulation distortion voltages occurring on the low-frequency side and the high-frequency side, and xcfx863L and xcfx863U are the phase components of the third-order intermodulation distortion voltages occurring on the low-frequency side and the high-frequency side.
In this case, both intermodulation distortions occurring on the low-frequency side and the high-frequency side can be suppressed for by generating a voltage VI given by the following equation with the lonearizer (for example, predistortion circuit) and inputting it to the power amplifier:
VI=AI(cos xcfx891t+cos xcfx892t)xe2x88x92BIL cos[(2xcfx891xe2x88x92xcfx892)t+xcfx863L]xe2x88x92BIU cos[(2xcfx892xe2x88x92xcfx891)t+xcfx863U]xe2x80x83xe2x80x83[Equation 2]
where AI is the amplitude component of voltages having angular frequencies xcfx891 and xcfx892 and BIL and BIU are the amplitude components of third-order intermodulation distortion voltages occurring on the low-frequency side and the high-frequency side in the predistortion circuit. There are relationships AO=AIxc2x7G, BOL=BILxc2x7G, and BOU=BIUxc2x7G, where G is the voltage gain of the power amplifier.
According to the conventional technique, the amplitudes and the phases of third-order intermodulation distortion occurring in the predistortion circuit cannot be controlled independently on the low-frequency side and the high-frequency side. That is, BIL, BIU, xcfx863L, and xcfx863U cannot be controlled independently.
No particular problems occur with the above conventional linealizer if the amplitude components of third-order intermodulation distortion voltages on the low-frequency side and the high-frequency side are the same and their phase components are also the same.
However, if the amplitude components and/or the phase components of third-order intermodulation distortion voltages on the low-frequency side and the high-frequency side are different from each other as in the case that a transmission signal has a wide bandwidth and the power amplifier performs a class-AB operation, a problem arises that a sufficient linearization (for example, sufficient suppressing distortion) cannot be obtained.
The present invention has been made in view of the above problems in the art, and an object of the invention is therefore to provide a predistortion circuit, a low-distortion power amplifier, control methods therefor, etc. in which at least one of the amplitudes and the phases of a distortion on the high-frequency side and the low-frequency side are controlled independently and that are therefore effectively used for a power amplifier having such an unbalanced distortion characteristic.
The 1st invention of the present invention (corresponding to claim 1) is a predistortion circuit comprising:
a divider for branching an input signal into plural branched signals;
a delay circuit for delaying one of the branched signals by a predetermined delay time;
a distortion generating circuit for receiving the other branched signal and for generating a distortion signal;
a vector adjustment circuit for varying an amplitude and a phase of the distortion signal; and
a combining circuit for combining an output signal of the delay circuit and an output signal of the vector adjustment circuit and for outputting a combined signal to circuit means as a subject of linearization to be provided downstream of and connected directly or indirectly to the combining circuit,
wherein the delay time of the delay circuit is set based on a phase difference of a distortion that would be generated by the circuit means if the vector adjustment circuit did not produce the output signal.
The 2nd invention of the present invention (corresponding to claim 2) is a predistortion circuit comprising:
a divider for branching an input signal into plural branched signals;
a delay circuit for delaying one of the branched signals by a predetermined delay time;
a distortion generating circuit for receiving the other branched signal and for generating a distortion signal;
an amplitude frequency characteristic adjustment circuit for varying an amplitude frequency characteristic of the distortion signal;
a vector adjustment circuit for varying an amplitude and a phase of the distortion signal that is output from the amplitude frequency characteristic adjustment circuit; and
a combining circuit for combining an output signal of the delay circuit and an output signal of the vector adjustment circuit.
The 3rd invention of the present invention (corresponding to claim 3) is the predistortion circuit according to 2nd invention, wherein the delay time of the delay circuit is set based on a phase difference of a distortion that would be generated by circuit means as a subject of linearization to receive an output signal of the combining circuit if the vector adjustment circuit did not produce the output signal.
The 4th invention of the present invention (corresponding to claim 4) is the predistortion circuit according to 1st or 3rd inventions, wherein the delay time of the delay circuit is set based on the phase difference in such a manner that a first delay time is so set that a difference between the first delay time and a second delay time substantially equivalent to or corresponding to the phase difference, where the first delay time is the delay time itself and the second delay time is a delay time of a signal that is input to the combining circuit via the distortion generation circuit and the vector adjustment circuit.
The 5th invention of the present invention (corresponding to claim 5) is the predistortion circuit according to 1st or 3rd inventions, wherein the delay time of the delay circuit is variable.
The 6th invention of the present invention (corresponding to claim 6) is the predistortion circuit according to 1st or 3rd inventions, wherein the delay time of the delay circuit is fixed at a predetermined value.
The 7th invention of the present invention (corresponding to claim 7) is a predistortion circuit comprising:
a divider for branching an input signal into plural branched signals;
a delay circuit for receiving one of the branched signals;
a distortion generating circuit for receiving the other branched signal and for generating a distortion signal;
at least two filter circuits for separating the distortion signal into distortion signals having different frequencies;
at least two vector adjustment circuits connected to outputs of the respective filter circuits directly or indirectly, for varying amplitudes and phases of the distortion signals that are output from the respective filter circuits; and
a combining circuit for combining an output signal of the delay circuit and combined output signals of the respective vector adjustment circuits.
The 8th invention of the present invention (corresponding to claim 8) is the predistortion circuit according to 7th invention, further comprising at least two amplitude frequency characteristic adjustment circuits for varying amplitude frequency characteristics of distortion signals that are output from the respective filter circuits, wherein the vector adjustment circuits are connected to outputs of the respective amplitude frequency characteristic adjustment circuits.
The 9th invention of the present invention (corresponding to claim 9) is the predistortion circuit according to any one of 1st, 2nd, 3rd, 7th, and 8th inventions, wherein the distortion generation circuit comprises a limiter amplifier.
The 10th invention of the present invention (corresponding to claim 10) is the predistortion circuit according to any one of 1st, 2nd, 3rd, 7th, and 8th inventions, wherein the distortion generation circuit comprises a diode.
The 11th invention of the present invention (corresponding to claim 11) is the predistortion circuit according to any one of 1st, 2nd, 3rd, 7th, and 8th inventions, wherein the distortion generation circuit comprises a zero-bias diode.
The 12th invention of the present invention (corresponding to claim 12) is the predistortion circuit according to any one of 1st, 2nd, 3rd, 7th, and 8th inventions, wherein the distortion generation circuit comprises:
a divider for branching an input signal into plural signals;
a delay circuit connected to one output side of the divider;
a circuit connected to the other output side of the divider and comprising a nonlinear device;
a vector adjustment circuit connected to an output side of the circuit comprising the nonlinear device; and
a combining circuit for combining an output signal of the delay circuit and an output signal of the vector adjustment circuit.
The 13th invention of the present invention (corresponding to claim 13) is a low-distortion power amplifier comprising:
a combining circuit for combining an input signal with another signal;
a power amplifier for receiving an output signal of the combining circuit;
a divider for branching an output signal of the power amplifier into plural branched signals;
a distortion extraction circuit for extracting a distortion signal from one of the branched signals;
an amplitude frequency characteristic adjustment circuit for varying an amplitude frequency characteristic of the distortion signal; and
a vector adjustment circuit for varying an amplitude and a phase of the distortion signal that is output from the amplitude frequency characteristic adjustment circuit,
wherein an output signal of the vector adjustment circuit is input to the combining circuit as said another signal and the other branched signal is output from the low-distortion power amplifier.
The 14th invention of the present invention (corresponding to claim 14) is a low-distortion power amplifier comprising:
a combining circuit for combining an input signal with another signal;
a power amplifier for receiving an output signal of the combining circuit;
a divider for branching an output signal of the power amplifier into plural branched signals;
a distortion extraction circuit for extracting a distortion signal from one of the branched signals;
at least two filter circuits for separating the distortion signal into distortion signals having different frequencies; and
at least two vector adjustment circuits for varying amplitudes and phases of the distortion signals that are output from the respective filter circuits,
wherein a signal obtained by combining together output signals of the vector adjustment circuits is input to the combining circuit as said another signal and the other branched signal is output from the low-distortion power amplifier.
The 15th invention of the present invention (corresponding to claim 15) is a low-distortion power amplifier comprising:
a combining circuit for combining an input signal with another signal;
a power amplifier for receiving an output signal of the combining circuit;
a divider for branching an output signal of the power amplifier into plural branched signals;
a distortion extraction circuit for extracting a distortion signal from one of the branched signals;
at least two filter circuits for separating the distortion signal into distortion signals having different frequencies;
at least two amplitude frequency characteristic adjustment circuits for adjusting amplitude frequency characteristics of the distortion signals that are output from the respective filter circuits; and
at least two vector adjustment circuits for varying amplitudes and phases of distortion signals that are output from the respective filter circuits,
wherein a signal obtained by combining together output signals of the vector adjustment circuits is input to the combining circuit as said another signal and the other branched signal is output from the low-distortion power amplifier.
The 16th invention of the present invention (corresponding to claim 16) is a control method for the predistortion circuit according to any one of 1st, 2nd, 3rd, 7th, and 8th inventions, comprising the steps of:
connecting a power amplifier to an output side of the predistortion circuit;
detecting a magnitude of a distortion signal generated by the power amplifier; and
controlling at least one of the amplitude frequency characteristic adjustment circuit or circuits, the vector adjustment circuit or circuits, and the delay time of the delay circuit so as to minimize the detected magnitude of the distortion signal.
The 17th invention of the present invention (corresponding to claim 17) is the control method for the low-distortion power amplifier according to any one of 13th to 15th inventions, comprising the steps of:
detecting a magnitude of the distortion signal that is output from the distortion extraction circuit; and
controlling at least one of the amplitude frequency characteristic adjustment circuit or circuits and the vector adjustment circuit or circuits so as to minimize the detected magnitude of the distortion signal.
The 18th invention of the present invention (corresponding to claim 18) is a linearized power amplifier comprising:
the predistortion circuit according to any one of 1st, 2nd, 3rd, 7th, and 8th inventions;
a power amplifier for receiving an output signal of the predistortion circuit;
a divider for branching an output signal of the power amplifier into plural branched signal;
detecting means of receiving one of the branched signals and detecting an amplitude and a phase of a distortion signal that is output from the power amplifier; and
control means of controlling at least one of the amplitude frequency characteristic adjustment circuit or circuits, the vector adjustment circuit or circuits, and the delay circuit of the predistortion circuit so as to minimize a distortion generated by the power amplifier based on an output signal of the distortion amplitude and phase detecting means,
wherein the other branched signal becomes at least one output signal of the linearized power amplifier.
The 19th invention of the present invention (corresponding to claim 19) is a feedforward amplifier having a predistortion circuit, comprising:
a divider for branching an input signal into plural branched signals;
a first vector adjustment circuit for varying an amplitude and a phase of one of the branched signals;
the predistortion circuit according to any one of claims 1st, 2nd, 3rd, 7th, and 8th inventions for receiving an output signal of the first vector adjustment circuit;
a first power amplifier for receiving an output signal of the predistortion circuit;
first distortion level detecting means of detecting a magnitude of a distortion component included in an output signal of the first power amplifier;
a first delay circuit for receiving the other branched signal;
a first combining circuit for combining an output signal of the first delay circuit and the output signal of the first power amplifier;
a second delay circuit for delaying the output signal of the first power amplifier;
signal level detecting means of detecting a magnitude of an output signal of the first combining circuit;
a second vector adjustment circuit for varying an amplitude and a phase of the output signal of the first combining circuit;
a second power amplifier for receiving an output signal of the second vector adjustment circuit;
a second combining circuit for combining an output signal of the second power amplifier and an output signal of the second delay circuit;
second distortion level detecting means of detecting a magnitude of a distortion component included in an output signal of the second combining circuit; and
control means of controlling the predistortion circuit, the first vector adjustment circuit, and the second vector adjustment circuit based on output signals of the first distortion level detecting means, the signal level detecting means, and the second distortion level detecting means, respectively,
wherein the control means repeatedly performs, in arbitrary order, a first control of controlling at least the predistortion circuit so as to minimize a distortion level detected by the first distortion level detecting means, a second control of controlling at least the first vector adjustment circuit so as to minimize a signal level detected by the signal level detecting means, and a third control of controlling at least the second vector adjustment circuit so as to minimize a distortion level detected by the second distortion level detecting means.
The 20th invention of the present invention (corresponding to claim 20) is the feedforward amplifier having a predistortion circuit according to 19th invention, wherein:
the first delay circuit is a variable delay circuit whose delay time is variable;
a variation amount of a delay time when each of the first vector adjustment circuit and the predistortion circuit was controlled is stored in the control means; and
the control means controls the predistortion circuit and the variable delay circuit as the first control, controls the first vector adjustment circuit and the variable delay circuit as the second control, and controls only the second vector adjustment circuit as the third control.
The 21st invention of the present invention (corresponding to claim 21) is a predistortion circuit comprising:
a divider for branching an input signal into plural branched signals;
a delay circuit for delaying one of the branched signals by a predetermined delay time;
a distortion generating circuit for receiving the other branched signal and for generating a distortion signal;
a vector adjustment circuit for varying an amplitude and a phase of the distortion signal; and
an amplitude frequency characteristic adjustment circuit for varying an amplitude frequency characteristic of the distortion signal that is output from the vector adjustment circuit;
a combining circuit for combining an output signal of the delay circuit and an output signal of the vector adjustment circuit.